Directional control valves for changing over the direction of hydraulic pressure from a hydraulic pump are incorporated in a hydraulic circuit, as means for controlling the operation of actuators, such as acceleration, deceleration and the like, in construction machines, agricultural machines and other various industrial machines.
Such directional control valves generally incorporate therein a spool which is manually controlled in its position by means of a hand lever, a pedal or the like in view of the necessity of manipulation under the operator's intention and economic reasons.
In a manually operated directional control valve of the type as mentioned above, a spool extends through a valve body, and is adapted to slide in a straight direction by means of a manipulating lever attached to one end of the spool, and a return spring is incorporated on the other end side of the spool, remote from the manipulating lever, for returning the spool to its neutral position (center valve position) when manipulation force is released from the lever. However, the directional control valve is sometimes required to be used in such a way that the spool valve is held, rather than the neutral valve position, at a predetermined offset valve position such as a spool position at which hydraulic oil is returned from an actuator into a tank while hydraulic oil discharged from a pump is subjected to neutral circulation. Accordingly, a detent device is used in the directional control valve for attaining this purpose.
The above-mentioned detent device incorporated in the directional control valve usually consists of an assembly including a return spring disposed in the outer wall of the valve body, as disclosed in Japanese Utility Model Laid-Open Publication No. 7977/1984 as a typical one. FIG. 8 illustrates a conventional detent device which comprises a front casing 4a receiving one end of a spool 2 and attached to a housing 1, and a rear casing 4b coupled in series to the front casing 4a through the intermediary of a partition ring. Further, in the front casing, there are disposed a return spring 11 for always urging the spool toward its neutral position, and a bolt 17 secured at one end to the spool 2 and extended from the front casing 4a into the rear casing 4b, the bolt 17 being formed therein with a deep stepped bore 170 extending axially from the rear end thereof, and being also formed in the outer peripheral wall of the rear end part thereof with a plurality of holes in which balls 6a are loosely fitted.
Further, a detent casing 18 formed on its inner surface with an annular projection 180 adapted to be engaged with the abovementioned balls is disposed in the rear casing 4b, and a small diameter bolt 22 is concentrically fitted in the bore 170, having a head 220 onto which a conical member 19 adapted to press the above-mentioned ball 6a is fitted slidably in the axial direction while a spring 21 is disposed between the conical member 19 and the bottom of the bore to urge the conical member 19.
In the above-mentioned arrangement, since the annular projection 180 adapted to retain the spool 2 is formed on the inner cylindrical surface of the detent casing 18 and since the balls 6a and the detent spring 21 are disposed in the detent casing 18, the detent device should inevitably have a large diameter and a long length due to the positional relationship among the above-mentioned parts. In particular, the large diameter causes the detent device to project from the valve body in the widthwise direction of the latter (the direction orthogonal to the direction of insertion of the spool). As a result, if there are used a plurality of directional control valves (section) which are fastened together with the use of, for example, bolts to form a stack type multiple directional control valve, dimensional adjustment with the use of spacers and the like is required, resulting in the problems of complicated working and oil leakage from the joint of the stack.
Further, the small diameter bolt 22 serving as both stopper for the conical member 19 and spring guide is inevitably used in addition to the detent casing 18 and the bolt 17 in order to obtain a detent function. Accordingly, the number of required parts is large. Further, comparatively large labor and time have been conventionally required for machining and assembly for the detent device since the bolt 17 is machined to form the bore and a female thread part therein with a high degree of accuracy and is studded with the small diameter bolt 22 highly concentric with the bore, and since the small diameter detent spring is assembled around the small diameter bolt 22, and therefore, it has been difficult to aim at reducing the cost of the detent device.